1. Field of Invention
This invention relates generally to wastewater stream recycling. In a specific respect, the invention relates to improvements in the handling of process blowdown by indirect contact heat exchange methods as secondary evaporators.
2. Discussion of Prior Art
The concept of this invention is applicable to all process blowdown streams, and not limited to only wastewater streams. However, by using steam boiler blowdown as a specific example, the concept of this invention can be more conveniently illustrated.
Besides process equipment dedicated to steam generation such as utility steam boilers, numerous other process applications of vaporizing water to generate steam have long been in practice. Example like waste heat boilers using kettle type reboilers; process coolers using deaerated water or steam condensate to generate steam to control process heat removal; dilution steam generation for use in cracking heaters using heat recovered from cracking heater effluent; and the list goes on. The common element in the above applications is the transfer of heat energy by the use of water/steam as the carrier and latent heat associated with phase change as the mechanism.
When water is evaporated to generate steam, the impurities in the water tend to accumulate in the liquid phase. The level of total dissolved solids (TDS) is generally controlled by blowdown. Too high a TDS level will cause fouling and ultimately lead to equipment failure. In steam generating boilers for example, a typical blowdown rate is around 5% of the water makeup rate. That rate is dependent on feed water quality and treatment chemicals used. For those skilled in the art would be familiar with calculation method using material balance equations. Briefly, a 5% blowdown rate results in a 20-fold increase (or 20 cycle) in concentration of the feed water TDS. That is because the 5% water leaving has to remove 100% of the TDS in the feed stream at steady state condition. At higher blowdown rate, say at 10%, cycle of concentration will drop to 10. Conductivity measurements, reflecting TDS, are commonly used to measure and control TDS levels by adjusting the rate of blowdown. This water outlet stream is often referred to as continuous blowdown. Intermittent blowdown in the xe2x80x9cmud drumxe2x80x9d is also routinely done so that settled solids can be removed periodically, say, once or twice a 12-hour shift. Anti-fouling boiler treatment chemicals, such as organic phosphates, are often added to keep the TDS in suspension instead of depositing onto boiler tube surfaces. The cost savings in reducing blowdown are used to justify the cost of such chemical addition.
The costs associated with blowdown have long been recognized by the processing industry. A common practice to recover some of the energy in the blowdown stream is by the use of flash drums and economizers. Briefly, when water at saturation temperature at the boiler operating pressure is let down to a lower pressure, say to a flash drum maintained at a lower pressure, a small portion of the water will turn into steam. The flashed steam can be used for lower temperature heating usage. The blowdown water can then be used to preheat incoming makeup water to further recover the heat energy in the effluent water before being sent to wastewater treatment facilities prior to discharge. Consequently, the true cost of blowdown includes the cost of water supply, boiler feed water treatment as well as the post treatment costs before discharge. Water treatment costs aimed at minimizing blowdown rate or higher cycles of concentration in the boiler is being justified accordingly, so says boiler treatment chemical manufacturers. The impact of reducing this blowdown stream, even by a small fraction, is enormous when all steam boilers and process steam generators currently in service are tallied up.
Naturally, it would be economically beneficial to reduce the blowdown rate but not at the expense of higher TDS or increased fouling potential. This invention provides a method of minimizing blowdown without the use and cost associated with additional chemical additives, further helping to improve the environment and shifting the economics towards less overall chemical additives to steam generating systems.
Accordingly, several objects and advantages of this invention are: The present invention focuses on opportunities to recycle process condensate so that process blowdown can be minimized. The present invention takes the approach of built-in operability so that the resulting apparatus can be serviced without process interruption. It is another object of this invention to improve the reliability of steam generating devices by allowing a reduction in average cycle of concentration in the evaporator through higher blowdown rates but without the usual costs associated with increased blowdown. It is another object of this invention to provide economic incentives to minimize the use of chemical treatment, its associated costs and impact on the environment. It is yet another object of this invention to reduce the net effluent flow to wastewater treatment facilities, directly impacting on environmental costs. It is yet another object of this invention to reduce the net use of fresh makeup water, also positively impacting on environmental costs.
Further objects and advantages of this invention will become apparent from a consideration of drawings and ensuing description.